code: def _cubic_coeff(signal):
      zi = -2 + sqrt(3)
      K = len(signal)
      powers = zi ** arange(K)
  
      if K == 1:
          yplus = signal[0] + zi * add.reduce(powers * signal)
          output = zi / (zi - 1) * yplus
          return atleast_1d(output)
  
      # Forward filter:
      # yplus[0] = signal[0] + zi * add.reduce(powers * signal)
      # for k in range(1, K):
      #     yplus[k] = signal[k] + zi * yplus[k - 1]
  
      state = lfiltic(
          1, r_[1, -zi], atleast_1d(add.reduce(powers * signal))
      )
  
      b = ones(1)
      a = r_[1, -zi]
      yplus, _ = lfilter(b, a, signal, zi=state)
  
      # Reverse filter:
      # output[K - 1] = zi / (zi - 1) * yplus[K - 1]
      # for k in range(K - 2, -1, -1):
      #     output[k] = zi * (output[k + 1] - yplus[k])
      out_last = zi / (zi - 1) * yplus[K - 1]
      state = lfiltic(-zi, r_[1, -zi], atleast_1d(out_last))
  
      b = asarray([-zi])
      output, _ = lfilter(b, a, yplus[-2::-1], zi=state)
      output = r_[output[::-1], out_last]
      return output * 6.0
  
file_path: scipy/scipy/signal/_bsplines.py
imports: from numpy import asarray, pi, zeros_like, array, arctan2, tan, ones, arange, floor, r_, atleast_1d, sqrt, exp, greater, cos, add, sin
  from ._spline import cspline2d, sepfir2d
  from ._signaltools import lfilter, sosfilt, lfiltic
  from scipy.interpolate import BSpline
label_desc: Use image node_id 9 for calling a global function with example usage: _cubic_coeff(signal) and returns: unknown, atleast_1d